The present invention relates to an electrostatic chuck plate for electrostatically holding samples, which is made of electrically conductive material or semiconductive material and which is used in processing or inspecting samples.
In processing or inspecting a semiconductor it is necessary to clamp and hold the wafer at a prescribed position. It is required that a wafer be held on a flat table, particularly in drawing a fine pattern on the wafer to thereby manufacture an integrated circuit comprising a number of transistors.
A mechanical chuck, a magnetic chuck, an electric chuck and a vacuum chuck utilizing a pressure difference between fluids are commonly used to clamp and hold samples. The electric chuck is useful in manufacturing semiconductor devices, because it is easy to operate and can hold a sample on a flat table.
The electric chuck, or more precisely an electrostatic chuck, utilizes mutual attraction between two capacitor plates which are electrically charged in the opposite directions. It comprises an electrode and a dielectric layer formed on the electrode. A sample made of electrically conductive material or semiconductive material will be attracted toward the electrode and will thus be held on the dielectric layer. Force F of attraction between the electrode and the sample largely depends on the physical properties of the dielectric layer. Force F is generally given by: ##EQU1## where .epsilon..sub.o is the vacuum dielectric constant of the dielectric layer, .epsilon..sub.s is the relative dielectric constant of the layer, S is the area of the contact surfaces of the sample and the electrode, V is the voltage applied on the layer and t is the thickness of the layer. Obviously, the greater .epsilon..sub.s is and the smaller t is, the lower may be the voltage V. The dielectric layer must be resistant to both wear and insulation or dielectric strength because a number of samples will be put on it, one at a time.
An electrostatic chuck plate having a dielectric layer made of mica, polyester or barium titanate is disclosed in British Pat. No. 144,321 and "Electrostatic Waferchuck for Electron Beam Microfabrication", George A. Wardly, Rev. Sci. Instrum., Vol 44, No. 10, October 1973, 1506. The dielectric layer is bonded to an electrode disc, using an adhesive agent. The adhesive agent can be hardly applied to form a layer of a uniform thickness. The dielectric layer will thus, in most cases, fail to have a flat surface even if it has a uniform thickness.
An electrostatic chuck plate having a dielectric layer made by an anodic treatment of material is disclosed in Japanese Patent Disclosure (Kokai) No. 55-145351. The dielectric layer is so thin that it will have cracks after many samples have been held on it, one at a time. Once cracks are made in it, the layer can no longer function as an insulator. In order to avoid this, an electrode material may be oxidized very much, thus forming a thicker dielectric layer. A thick dielectric layer has a rough surface, however, and the area of the contact surface of the layer and a sample are extremely small. In this case, the force of attraction between the sample and the electrode will be reduced. Further, the porous portion of the thick dielectric layer has a relative dielectric constant of about 1, whereby the relative dielectric constant of the layer is small. Consequently, the force of attraction between the sample and the electrode will be far smaller than desired.